An ultrapure-water production system for producing ultrapure water from raw water such as city water, groundwater, or industrial water includes a pretreatment subsystem, a primary pure-water production subsystem, and a secondary pure-water production subsystem. The pretreatment subsystem includes a coagulation apparatus, a floatation apparatus, a filtration apparatus, a clarification-membrane apparatus, and the like. The primary pure-water production subsystem includes one or more apparatus selected from an active-carbon adsorption column, an ultraviolet (UV) oxidation apparatus, a chemical oxidation apparatus, a degassing apparatus, and the like and a desalting apparatus. The desalting apparatus includes one or more devices selected from a reverse-osmosis (RO) membrane separation device, an electrodeionization device, and an ion-exchange device (a mixed-bed ion-exchange device or an ion-exchange pure water device). The secondary pure-water production subsystem includes device units similar to those included in the primary pure-water production subsystem as needed. A common second pure-water production subsystem includes a low-pressure UV oxidation device, a mixed-bed ion-exchange device, and an ultrafiltration (UF) membrane separation device.
Among the above device units, the RO membrane separation device, the electrodeionization device, and the mixed-bed ion-exchange device are responsible for the desalting of raw water; the RO membrane separation device and the UF membrane separation device are responsible for the rejection of microparticles contained in raw water; and the RO membrane separation device, the ion-exchange pure water device, and the low-pressure UV oxidation device are responsible for the rejection of TOC components.
Strict water quality standards, such as a boron concentration of 1 ppt or less, have been applied to the production of ultrapure water.
It has been proposed to use a high-performance electrodeionization device capable of achieving a high boron rejection (e.g., “KCDI-UPz” produced by Kurita Water Industries Ltd.) for deionization of water with a combination of an RO membrane separation device. However, the boron rejection of the high-performance electrodeionization devices is about 99.9% at the highest. Therefore, when water to be treated having a boron concentration of about 20 ppb for example is treated with an RO membrane separation device to produce RO permeate water having a boron concentration of about 10 ppb, and the permeate water is then treated with an electrodeionization device having a boron rejection of 99.9%, the boron concentration in the treated water (deionized water by the electrodeionization device) is 10 ppt at the lowest. In other words, it is not possible to produce treated water having a boron concentration of 1 ppt or less.
Common electrodeionization devices include a cathode, an anode, and a plurality of cation-exchange membranes and anion-exchange membranes that are arranged alternately between the cathode and the anode so as to form concentrating compartments and desalting compartments that are arranged alternately. The desalting compartments are filled with an ion-exchange resin. In some electrodeionization devices, the concentrating compartments are also filled with an ion-exchange resin.
Ion-exchange resins filled in desalting compartments and concentrating compartments included in the electrodeionization devices used in the related art have a particle size of about 500 to 600 μm. The uniformity in the particle size of the ion-exchange resin has not been considered.
Desalting compartments of an electrodeionization device in Examples of Patent Literature 1 are filled with an anion-exchange resin “DIAION (registered trademark) SA10A” (average particle size: 540 μm) produced by Mitsubishi Chemical Corporation and a cation-exchange resin “DIAION (registered trademark) SK1B” (average particle size: 620 μm) produced by Mitsubishi Chemical Corporation.
Patent Literature 2 proposes a deionized-water production system including desalting compartments filled with a mixture of groups of ion-exchange resin particles which have different uniform particle sizes. The particle size of a group of ion-exchange resin particles which has the largest uniform particle size is 1.5 times or more the particle size of a group of ion-exchange resin particles which has the smallest uniform particle size. Although it is described in Patent Literature 2 that the particle size of the group of ion-exchange resin particles which has the smallest uniform particle size is 30 to 600 μm, the ion-exchange resin actually used in Examples of Patent Literature 2 is a mixture of a cation-exchange resin having an average particle size of 630 μm, a cation-exchange resin having an average particle size of 220 μm, and an anion-exchange resin having an average particle size of 575 μm at a ratio of 25:22.5:52.5 (weight ratio).
It is described in Examples of Patent Literature 3 that a molded article of an ion-exchange resin is filled in desalting compartments. The article is formed by molding a cation-exchange resin having an average particle size of 600 μm and an anion-exchange resin having an average particle size of 550 μm with a binder.
Patent Literature 1: Japanese Patent Publication 2001-113281A.
Patent Literature 2: Japanese Patent Publication H10-258289A
Patent Literature 3: Japanese Patent Publication 2002-1345A